Proposed constitutional reform would give each nation and region of the UK full sovereignty over its own affairs.

The governance of England – Scotland – Wales and Northern Ireland should be reinvented within a new voluntary union in a bid to save the UK from disintegration – an independent all-party group of experts will argue this week.

The Constitution Reform Group – convened by former Conservative cabinet minister Lord Salisbury – is to make the the case for radical constitutional change in the UK by claiming the need has been boosted by the vote to leave the European Union.

Their proposals say the existing union should be replaced with fully devolved government in each part of the UK – with each given full sovereignty over its own affairs.

The Westminster parliament – the group says – should then be reduced to 146 MPs.

The individual nations and regions of the UK would then be encouraged to pool sovereignty to cover the matters they wish to be dealt with on a shared basis.

The proposals say they ‘start from the position that each of England – Scotland – Wales and Northern Ireland is a unit that both can and should determine its own affairs to the extent that it considers it should; but that each unit should also be free to choose to share through an efficient and effective United Kingdom – functions which are more effectively exercised on a shared basis’.

The new construction suggests a complete reversal of the UK’s current constitutional arrangement in which all sovereignty formally rests in the centre and is then devolved to regions on a piecemeal basis.